Laser amplifier device



July 30, 1968 E. SNITZER 3,395,356

LASER AMPLIFIER DEVICE Filed Oct. 26, 1966 3,395,356 LASER AMPLIFIERDEVICE Elias Snitzer, Sturbridge, Mass., assignor, by mesne assignments,to American Optical Company, Southbridge, Mass., a corporation ofDelaware Continuation-impart of application Ser. No. 411,203, Nov. 16,1964. This application Oct. 26, 1966, Ser. No. 595,291

Claims. (Cl. S30-4.3)

ABSTRACT 0F THE DISCLOSURE A laser device wherein within the hostmaterial is placed ions of one kind in a sufficient concentration toprovide repopulation of selected ions by transfer of energy to selectedions from neighboring ions after the selected ions have lased, or a hostmaterial containing one type of ions and another type of ydifferentneighboring ions both ions being in such a concentration that energy canbe transferred to selected ions, once the selected ions have lased, bythe different neighboring ions, thus providing an improved lasermaterial which substantially reduces the Irefractory time period betweensuccessive laser pulses.

This invention is a continuation-in-part of S.N. 411,203, tiled Nov. 16,1964, for Laser Device by Elias Snitze-r, inventor, and now abandoned.

This invention relates to Iamplitic'ation devices and more particularlyto laser amplifiers and materials use-ful therein for increasing therepetition rates of outputs therefrom by shortening the refractoryperiod of sai-d devices.

In order to describe the invention, it is important first to briefly setforth the operating characteristics of lasers, sometimes referred to asoptical masers. Lasers are lightamplifying or light-producing devicesand are specifically adapted Ito provide an output of high-intensity,coherent, monochromatic light. Such light is produced in a laser (anacronym for light amplification by stimulated emission of radiation) byphotonic emission from the activator ions or atoms of a body composed ofa so-called laser host maiterial, which is disposed coaxially Within aresonant cavity. These atoms, which are' in a positive temperaturestate, absorb a quantum of light or energy from a pumping source, whichis at a frequency corresponding to the difference in energy between twoof the energy levels of the atom. The atoms are thereby pumped orexcited to a high energy level and a negative temperature state ofpopulation inversion, from which they rapidly and spontaneously (butnon-radiatively) relax to a more stable intermediate level (still abovethe original level). From this intermediate level, the atoms`spontaneously relax to the original level with an attendant fluorescentemission.

The liuorescen't energy emitted by the spontaneously relaxing atomspasses through the resonant cavity to the ends thereof Iand is thenreflected Iback and forth through the cavity to excite other atoms atthe intermediate energy le'vel to induce them to undergo emissivetransitions downward, producing more light yand augmenting thebidinectionally reflected light to induce still further emissivetransitions from the intermediate level population. In this way, arising pulse of bidirectionally reflected light quickly develops withinthe cavity, reaching a quantitatively large value as the e-missivetransition of atoms from the inrtermediate level population becomesmassive. Light of high intensity is, accordingly, cre-ated in one or asuccession of light pulses while the pumping source is active, theaction continuing until depletion of the intermediate level populationrestores the -laser 'body -to a positive temperature state. To permitemission of a portion of this bidirecnited States Patent O 3,395,356Patented July 30, 1968 ICC tionally reflecting light pulse or pulsesfrom the laser cavity, one reflective end of the cavity is madepartially transmissive and Athe fraction of light escaping therethroughconstitutes the laser output.

Since the stimulated transition downward from the intermediate energylevel occurs in a significantly shorter time period than the `timeperiod necessary for establishment of the negative temperaturedistribution of energy levels `of the atoms, it may -be seen that thetime between pulses is largely determined -by the ability of the deviceto attain the negative temperature state in a short time period. Theinvention described 'herein is more clearly understood by analogy ofthis time period for attainment of a negative temperature distributionto the Irefractory period of a neuristor device. A neu-ristor, 'asdescribed Iby H. D. Crane in an article entitled The Neuristor appearingin Principles of Self Organization edited by Foerster and Zopf, pages403 through 415 (Pergamon Press, New York, 1962), is a device stimulatedto propagate a signal down its line or channel by the application ofenergy thereto. Once the neuristor line is so stimulated, it propagatesa signal at a uniform velocity and With-out attenuation, which producesa refractory condition which may be defined as the time period necessaryfor each portion of the line, following propogation therethrough, untilthe line is in condition to propagate another pulse. To complete thisanalogy, it maybe said that the refractory period of the laseramplification device is the time taken for atoms in the laser body toattain a negative temperature distribution state, after such atoms havemade a transition to the lower energy level, in order to permit anotherpulse to be propagated. Shortening of this refractory period in thelaser would of course, allow a higher frequency or greater repetitionrate in the output of the laser device, and such high frequencies allowfor Iadvantages such as increased bandwidths for signal communications.

Accordingly, the present invention is directed towards reducing therefractory period by providing laser materials in such concentrationsabsolutely and relative to each other, that the intermediate energylevel or metastable level of the laser atomic distribution is almostperpetually populated |by energy transfer between the closely packedions of the material, thereby providing a quickly -recurring negativetemperature state and a high repetition rate capability for the laserpropagation.

These and other objects of the invention are accomplished in oneillustrative embodiment by a solid laser device with a shortenedrefractory period provided by the transfer of energy between metastablestates of two liuorescent materials, lsaid energy transfer being enabled-by high concentrations of at least one material. In this way, when astimulated emission occurs in one material causing a reduction in thepopulation of atoms in the metastable energy level -for that material,that metastable level is quickly replenished by the transfer of energyfrom the atoms existing at the metastable level of the other material.

Another embodiment lfeatures a single fluorescent material provided inhigh enough concentration, so that energy may be non-radiativelytransferred between metastable levels of adjacent ions or atoms. Thistransfer of energy atoms is referred to herein as cross-relaxation andoccurs when the metastable level for the material is depleted and theatoms are quickly raised to an energy level suflicient to replenish thedepleted metastable level by transfer of energy between neighboringatoms or ions.

Other objects, features and embodiments are contemplated and will beapparent from the following more detailed description with. reference tothe accompanying drawings, wherein:

FIG. 1 is a graphical representation of the operating characteristics ofa neuristor, useful in comprehendingV the operating characteristics ofthe present inventionj FIG. 2 is a representation of an amplifier deviceaccording to the present invention;

FIG. 3 is a plot of the number of ions versus the various wavelengths atwhich their fluorescent energy levels exist. This is the so-calledinhomogeneous broadening of the line, which is produced by the glassyhost in which the ions or atoms are contained;

FIG. 4 is a representation of energylevels for different groups of ionsof trivalent ytterbium in the amplifying device of FIG. 2, with adepiction of the inter-relationships of the ions by means ofcross-relaxation; and

FIG. 5 is an energy level diagram of the ions of trivalent neodymium andtrivalent ytterbium and a depiction of the energy transfer between themetastable levels of each.

Referring first to FIG. 1, there is shown a representation of theoperating characteristic 12 of a neuristor line. A neuristor device canbe likened to a re-usable fuse, wherein its normal state is an unburntcondition. After a pulse has passed, the neuristor goes through aburning period, after which time it is in a refractory condition whichincludes the time it is burnt and the time period 13 necessary for it tobe made re-usable by returning to the unburnt condition. The analogybetween a device of the present invention is best described by directanalogy with the previously mentioned unburnt, burningf and refractoryperiods or conditions of the neuristor. For instance, the unburntcondition of -a neuristor is comparable to the negative temperaturedistribution state of a laser material, when the metastable energy levelis sufficiently populated `for stimulated downward transition to occur.The burning period can be likened to the transition downward or thestimulated emission of radiation by ions of the fluorescent material.This transition downward causes a burnt condition or, in other words, apositive temperature distribution. The positive temperature distributionand the ensuing period during which the laser device is repumped is arefractory period, or the time it takes for re-establishment of thenecessary inversion after dumping of ions or atoms from the metastableenergy level. This period is significant, since a high repetition rateoutput is made impossible by the `fact that a succeeding laser outputpulse cannot be generated usually for one to ten microseconds.Therefore, the shortening of this refractory period is the primaryobject and subject matter of the invention described herein.

A laser amplifier device is depicted in FIG. 2 as comprising anamplifier 14 with a central core 16 of laserable material and a cladding18 concentrically surrounding said core. The cladding 18 is of atransparent glass whose index of refraction is less than the index ofrefraction of the core 16. In this way, the cladding serves to retainlight energy within the core for propagation therethrough. The ends ofthe core are of low reflectance, since oscillations are unnecessary tostimulate downward transitions, which are caused by the output of source22. Also, reflections could serve only to increase oscillation with adetrimental effect on gain. A source of pumping energy 20 provides anenergy source for the establishment and re-establishment of a negativetemperature distribution of the core material 16. The monochromatic orsingle frequency energy source 22 provides pulses of energy to one endof the amplifier device, with that source comprising a laser device`such as that described in the introduction of this specification. Eachof the pulses from the energy source 22 is propagated and amplifiedthrough the amplifier device Iby either the cross-relaxation of ions oratoms of a single solid fluorescent material or by energy transferbetween ions of two solid fluorescent materials, as will be explainedmore completely with reference to FIGS. 4 and 5. It should -beunderstood that the immediately preceding description with reference tothe use of a single frequency source 22 for a laser amplifier islikewise applicable and extendable to the use of a single frequencypropagating means such as a filter in the cavity of a laser oscillatorto produce the same result; that is, the propagation of inversion-depleting stimulated light in a narrow enough band of wavelengths sothat only the emitting ions will be depleted leaving energy transferringions still in an inverted state to transfer their energy to the depletedions.

The problem of lessening a protracted refractory period is graphicallydescribed with its general solution by reference to the plot of FIG. 3,which shows a bell-shaped curve 24 of the relationship between thenumber of ions and the wavelength of light emission from those ions.Curve 26 shows the area of depleted population of ions at the wavelengthy', said depletion being caused when a pulse has passed and radiation isstimulated to be emitted as a laser output. This area 26 must bere-populated before a succeeding laser output can be achieved, and aspreviously described, one way of achieving such re-population is byre-pumping with energy source 20. However, it is recognized that if asufficient concentration of ions of a single solid fiuorescent materialis provided for the amplifier core 16 of FIG. 2, a transfer of energy,or cross-relaxation, between neighboring ions of that material canaccomplish the re-inversion necessary for subsequent radiative output ata much quicker rate. The wavelength of energy emission from theseneighboring ions is depicted in lFIG. 3 by curves 28 and 30.Cross-relaxation tends to re-establish the population of ions within thearea circumscribed by curve 26, since the curves for the neighboringions 28 and 30 sufficiently overlap that area. Therefore, if the ions ofarea 26 have been dumped to ground level after a pulse has passed, theenergy transferred from the neighboring ions will serve to re-establishthe negative temperature distribution of ions within the area 26. A veryfast repetition rate of output pulses from the laser can thereby beobtained.

Similarly, two fluorescent materials having substantially identicalwavelength separation between the metastable energy state and theterminal state for their respective ions, can be made to accomplish thesame result as that provided by the single fluorescent material andgraphically described in FIG. 3.

FIG. 4 represents the energy levels of trivalent ytterbium in variousygroups of ions or atoms of the amplifier device 14. The upper ormetastable energy level E2' for the three ion groups Ybl, Ybz, and Yb3in FIG. 4, represents the 21:'5/2 state and the lower or ground energylevel E0 for the ion groups represents the ZFq/Z state in spectroscopicnotation. Atoms in the core 16 of the amplifier device of FIG. 2 areexcited from the ground level E0 to the metastable level E2' by thesource of pumping energy 20. The atoms in the excited or metastablestate subsequently go through an energy transition 32 to the groundstate E0', stimulated by the output of source 22, and radiate energy ofa frequency corresponding to the energy differences between these twostates. This downward transition of atoms from the metastable state tothe ground state in this first group of ions Ybl causes a depopulationof the metastable level E2' for those ions. However, cross-relaxation ofenergy from neighboring ions in the metastable level for Ybz ions can bemade to re-establish a negative temperature distribution for ions Ybl insubstantially less than one microsecond, which allows a subsequentoutput to be quickly enabled. Likewise, the metastable level of Ybz ionsis quickly re-populated by a transfer of energy from neighboring Ybaions. By this means of cross-relaxation, the metastable energy levels ofall ions are quickly re-established after dumping and furthermore,re-pumping by source 20 is all the time causing a re-inversion alongtransition lines 34, 36 and 38. In this embodiment, a concentration ofapproximately 15 weight percent Yb2O3 accomplishes the result, sincecross-relaxation is enabled by the proximity of the ions. (Aconcentration of 5-50 weight percent will be satisfactory, lbutapproximately 15 weight percent is preferred.)

An energy transfer is also used to the same advantage between twodifferent solid uorescent materials, as is shown in FIG. 5. In thatfigure, a neodymium material (Ndst) is shown to have a number of energylevels Ey- EG, representative of its energy level possibilities, and theytterbium material (Yb3+) is shown to have three levels En', E1', E2'.(In spectroscopic notation, the metastable level of neodymium is 4F3/2and the levels E3, vE2 and E1 are respectively 4113/2, 4111/2, 419/2.)An amplifier using these two materials in heavy enough concentrationwill operate with the pumping energy from source 20 causing a transitionupward of energy state by ions or atoms in the ground state. Therefore,the energy level transitions are from El of neodymium and E' ofytterbium to either energy levels Eli-E6 of neodymium or E2 ofytterbium, respectively. Any neodymium atoms attaining energy levels Eor E6 will subsequently go through a downward transition of energylevels, non-emissively, to metastable energy level E4. In this way, themetastable levels of neodymium and ytterbium are populated suiciently toestablish a negative temperature distribution condition, so that astimulated downward transition of ytterbium ions will dump the atomsfrom the metastable state E2. However, the fact that metastable energylevel E2 of Yb3+ is slightly lower than the metastable energy level E4of Nd3+ will enable a transfer of energy between these levels, therebyre-populating the metastable level E2. By this energy transfer, theytterbium is quickly re-established in a negative temperature state sothat a succeeding stimulation will cause a laser output in the amplifierwithout a protracted refractory period for the atoms previously dumpedto the E0' level. For accomplishment of this energy transfer betweenmetastable energy levels of different solid fluorescent materials,concentrations of approximately 5 weight percent of the oxide ofneodymium and 6 weight percent of the oxide of ytterbium is used andrecommended, even though concentrations of IAO to 10 weight percent ofthe oxide of neodymium and 1 to l0 weight percent of the oxide ofytterbium will enable a substantial increase in the repetition rate.

As an alternate embodiment, the E1' level of ytterbium can be used asthe terminal level as is shown in IFIG. 5, since it is a part of a splitground level of that material, and by the use of liquid nitrogen orother means, level El' is depleted in population in comparison to thelower of the two split levels. The negative temperature is thenestablished between energy levels E2 and E1' rather than E2' and E0 inlboth the cross-relaxation and energy transfer devices and thereby morereadily established. Also, it is possible to use gaseous laser materialsaccording to the invention as described herein.

I claim:

1. The new amplifier use of a laser device having an elongated body of ahost material containing laserable ions having a characteristicinhomogeneously broadened line width, said body having low reflectiveends and means for introducing pumping energy into said bodycharacterized in the following steps: (a) selecting the concentration ofsaid ions absolutely and relative to each other so that the intermediateenergy level or metastable level of the laser atomic distribution may bealmost perpetually populated by energy transfer between closely-packedions within said host material whereby transfer of energy between saidions can take place and an inversion of population at variouswavelengths within said iuhomogeneously broadened line width beestablished, (b) applying pumping energy to said ions to cause suchinversion of population, (c) introducing energy within said device assubstantially single frequency inversion depleting light at a wavelengthwithin said broadened line width for stimulating depletion of saidinversion in a selected one of said ions only with an attendenthomogeneous line width which is substantially narrower than saidinhomogeneously broadened line width, (d) immediately thereafterreinverting the energy state of said selected ions by energy transferfrom other than selected ions at a greater rate than the rate iat whichsaid selected ions can be re-inverted by only applying pumping energy tosaid device, and (e) again applying pumping energy to at least the otherof said ions.

2. The invention according to claim 1 wherein said laser-able ions areall trivalent ytterbium ions included within said host material in aconcentration of 5 to 50* weight percent of the oxide and wherein saidtransfer of energy is by cross-relaxation.

3. The invention according to claim 2 wherein said trivalent ytterbiumions lare included within said host material in a concentration of l5weight percent of the oxide.

4. The invention according to claim 1 wherein said laserable ionscomprise trivalent neody-mium and trivalent ytterbium ions includedwithin said host material in a concentration -of 1/10 to 10 weightpercent and 1 to l()l weight percent, respectively, of the oxide andwherein said transfer of energy is by energy transfer between said ions.

5. The invention according to claim `4 wherein said trivalent neodymiumand trivalent ytterbium ions are included within said host material in aconcentration of 5 weight percent and 6 weight percent, respectively, ofthe oxide.

References Cited UNITED STATES PATENTS Peterson et al.: 1964, pp.2011-204.

ROY LAKE, Primary Examiner. D. R. HOSTETTER, Assistant Examiner.

Applied Physics Letters, June 15,

